Vibrating microtome with automated measurement of vertical runout

ABSTRACT

A vibrating microtome and measuring device for measuring the transverse offset of a vibrating knife comprises a light barrier into which the knife is placeable and a control application signal (pklo, pkhi) that describes the time course of the vibration of the knife and an electronic measurement system of the measuring device measures the coverage of the light barrier as an oscillating signal (tpm), and determines the transverse offset from the signal values at points in time that are defined by the control application signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German patent application 10 2006041 208.7 filed Sep. 2, 2006, which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a device for measuring the verticalrunout in a vibrating microtome. More precisely, the invention relatesto a vibrating microtome in which a knife is configured, in particularduring a sectioning operation, to vibrate in a direction parallel to asection plane and substantially (i.e. within alignment accuracy values)parallel to a cutting edge of the knife, and to a pertinent measuringdevice that, for measurement of the transverse offset of the cuttingedge in the context of its lateral vibratory motion as a consequence ofa potentially present inclination of the cutting edge with respect tothe section plane, comprises a light barrier into which the cutting edgeis placeable, the light barrier being oriented in a direction parallelto the section plane and the cutting edge partially covering the lightbeam of the light barrier; the fluctuation over time of the measuredsignal derived from the light barrier, which fluctuation is occurringbecause of the vibration of the knife, is used to determine thetransverse offset.

BACKGROUND OF THE INVENTION

Vibrating microtomes in which the cutting edge performs an oscillatinghorizontal motion along the direction of the cutting edge, while thematerial being sectioned advances in the other horizontal direction andis thus sectioned along a horizontal sectioning surface, are well known,for example from the Applicant's DE 196 45 107 C2 and DE 20 2004 007 658U1. Vibrating microtomes of this kind are used in particular to sectiontissue specimens in liquids (buffer solutions), for example braintissue, or other materials of low plastic stability and/or gel-likeconsistency. In one usual geometry, the sample is fed forward vertically(Z axis) and stepwise from bottom to top. During an individualsectioning operation, the knife moves at the sectioning speedhorizontally (X axis) with respect to the sample. In that context, itvibrates substantially parallel to the cutting edge in a vibrationdirection that is perpendicular (Y axis) to the other motion directions,the vibration frequency being typically on the order of 100 Hz, forexample in the range from 90 to 100 Hz. Because of the tolerances of theknife holder and knife, however, it is inevitable that the knife edgedoes not move exactly parallel to the vibration direction. A knife thatis clamped in obliquely produces, because of the vibration, acorresponding motion in the Z direction; this transverse vibratoryoffset (i.e. perpendicular to the section plane, thus extending in the Zdirection in this case) is also referred to as vertical runout. Theconsequence of a vertical runout is that the sections exhibit awave-like pattern.

An electrical control system of a vibrating microtome is described inthe article “Patch-clamp recording in brain slices with improved slicertechnology,” Pflügers Arch—Eur. J. Physiol. (2002) 443:491-501 by J. R.P. Geiger et al., who propose a measuring head (referred to as a“vibroprobe”) as an aid to determining the transverse offset occurringin the context of knife oscillation. The measuring head operates with anIR light beam that is emitted by an LED and detected in a photodiode,and measures the magnitude, or the change over time in said magnitude,of the (partial) coverage of the light beam by the cutting edgepositioned in the beam path. The vertical runout occurring as aconsequence of the oscillating motion of the cutting edge thus yields anoscillating output signal whose vibration amplitude is to be minimizedby appropriate manual adjustment of the alignment of the cutting edge.By means of a setting screw (for tilting the knife), the knife edge isaligned parallel to the vibration direction and the vertical runout ofthe knife is thus reduced to a minimum. This operation of aligning theknife edge is time-consuming and cumbersome, not least because pivotingof the knife is generally associated with a realignment of the Zposition. A rapid alignment operation, on the other hand, is of greatadvantage, given that the samples are short-lived and must be processedquickly.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to indicate a way to simplifyand speed up alignment of the knife in terms of vertical runout, and toconfigure said alignment reliably for the entire range of possiblevibration amplitudes and frequencies of the knife.

This object is achieved by an electronic measurement system, provided ona measuring device of the kind described initially, that according tothe present invention is configured to accept at least one controlapplication signal that describes the time course of the vibration ofthe knife, and to perform the determination of the transverse offset ofthe cutting edge based on the values of the measured signal derived fromthe light barrier at points in time which are determined from saidcontrol application signal. The control application signal is preferablygenerated on the part of the vibrating microtome on the basis of one ormore signal(s) derived from the motion of the knife or of the knifeholder.

As a result of this manner according to the present invention ofachieving the object, which provides for the use of a constant-phasesignal to define the measurement points in time, the accuracy andrepeatability of the measurement of the transverse offset (verticalrunout) can be substantially increased. It is furthermore easier, orindeed possible at all, to eliminate interfering influences.

A preferred embodiment that additionally simplifies the measurementoperation provides for the at least one control application signal todefine the position in time of the vibration maxima of the knife, andfor the electronic measurement system to derive, from the measuredsignal, values that correspond to the transverse positions of thecutting edge at times of opposite vibration maxima, and to determine thetransverse offset from the difference between those values.

The measuring device can preferably be embodied as a unit detachablefrom the vibrating microtome, the electronic measurement system beinghoused in the measuring device. This simplifies operation as well as theaccessibility of the sample during the sectioning operation.

The measuring device, especially when it is realized as a detachableunit, can favorably generate a signal that describes the magnitude ofthe measured transverse offset, and convey said signal to the vibratingmicrotome so that it can be displayed, for example, on a displayassociated with the vibrating microtome.

It is additionally useful if the light barrier lies in a directionparallel to the (in this case merely prospective) section plane.

It is furthermore advantageous, in order to preclude interferinginfluences, if the electronic measurement system is additionallyconfigured to adjust the intensity of the light beam of the lightbarrier prior to a determination of the transverse offset, namely withthe cutting edge in a position completely outside the light barrier,wherein a utilization exceeding 90%, e.g. 95%, of the modulation rangeof the detector element of the light barrier is established.

In corresponding fashion, the stated object is achieved by a vibratingmicrotome of the kind cited initially having an electronic controlsystem which is configured to generate, from a vibration signal derivedfrom the vibratory motion of the knife, at least one control applicationsignal that describes the time course of the vibration of the knife; andis furthermore configured, for the purpose of a measurement of thetransverse offset of the cutting edge in the context of its lateralvibratory motion due to a potentially present inclination of the cuttingedge with respect to the section plane, to deliver said controlapplication signal to a measuring device that is provided on thevibrating microtome and has a light barrier into which the cutting edgeis placeable.

Advantageous refinements of the vibrating microtome correspond, mutatismutandis, to the refinements of the measuring device that are discussedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further advantages, is further explainedbelow with reference to a non-limiting exemplifying embodiment that isdepicted in the attached drawings, in which:

FIG. 1 is a perspective view of a vibrating microtome having a verticalrunout measuring head installed;

FIG. 2 is a sectioned view through the measuring head and the vibratinghead along the vertical center plane of the vibrating microtome of FIG.1;

FIG. 3 shows the control panel of the vibrating microtome; and

FIG. 4 is a block diagram showing the control system of the vibratingmicrotome and of the measuring head.

DETAILED DESCRIPTION OF THE INVENTION

The exemplifying embodiment presented below relates to a vibratingmicrotome in which a vertical runout measuring device in the form of ameasuring head is installed instead of the sample holder. Adetermination of the linear modulation range of the detector element iscarried out on the part of the measuring head, in the lowered position,at the beginning of each measurement, and the intensity of the lightbarrier of the transmitting element is set in such a way that the(uncovered) detector element is operated close to the upper limit of itslinear modulation range. The measurement itself is performed in themiddle of the linear modulation range, typically at 50% occlusion by theknife. The result of this operation is that each measurement operationis individually calibrated and remains largely independent of long-terminterfering influences (temperature, extraneous light, component drift).

The measuring head as a whole does not require any adjusting elementthat would need to be manually equalized. According to the invention,for the measurement operation the measuring device is supplied, by theelectronic system of the microtome, with a signal that describes thevibration operation of the knife over time, i.e. defines the exactlocations in time of the maximum left and right extension of thevibrating knife. The basis for this control application signal isconstituted by a digital measurement of the period of the knifevibration, with the aid of a measurement of the zero crossings of thefed-back knife position signal of the vibratory drive. This guaranteesthat the resolution of the time measurement is to a very large extentindependent of the amplitude and frequency of the vibration. The controlapplication signal comprises signal pulses respectively one-quarterperiod before and after a zero crossing. Sampling of the actual value ofthe vibration amplitude is likewise accomplished by means of thesesignal pulses. This ensures that the determination of the knifeobliquity is always performed synchronously with the knife vibration.Because the point in time of the measurement is highly precise, themeasurement result is stable over a small fluctuation range and cantherefore be determined very accurately: resolutions in the order ofmagnitude of 0.1% or less have been achieved.

Construction

FIG. 1 shows a vibrating microtome 1 that is based on the vibratingmicrotome of J. R. P. Geiger et al. in terms of its external layout andmechanical principle, but whose electronic control system is improved inaccordance with the invention, as will be explained below with referenceto FIG. 3. Vibrating microtome 1 comprises, in a manner known per se, avibrating head 2 that, as also shown in FIG. 2, is positioned in theform of an extension arm over the material to be sectioned (sample andsample carrier, not shown) together with its holder 4; in order tomeasure and compensate for the vertical runout, in place of the sampleholder a vertical runout measuring head 3 is installed on holder 4 bymeans of a clamping mechanism actuable via a clamping lever 4 a.

Vibrating head 2 comprises a knife holder 5 in which a knife 6 is heldin fixedly clamped fashion. In the exemplifying embodiment depicted, thesection plane extends horizontally, and sections that may optionallyfollow one another proceed vertically one above another. For thispurpose, vibrating head 2 and holder 4 (together with measuring head 3)can be moved vertically (Z axis) with respect to one another; the holdercomprises for this purpose, for example, a stepping motor (not shown) inthe lower region of vibrating microtome 1. By means of a permanentmagnet+coil+spring arrangement (not shown; cf. in this regard thearticle by J. R. P. Geiger et al.) housed in vibrating head 2, avibratory motion proceeding in a horizontal-lateral direction (Y axis)is imparted to knife holder 5 together with knife 6. Vibrating head 2can be displaced in the horizontal longitudinal direction (X axis) bymeans of a DC motor (not shown); in addition, a DC motor can likewise beprovided in the sample carrier (not shown), which motor serves for acontrollable, uniform motion of the sample in the X direction during thesectioning operation while the vibrating head remains stationary in theX direction.

It is not excluded that the three aforesaid directions X, Y, Z can, ifnecessary, also be oriented differently in other embodiments than in theexemplifying embodiment shown; as is immediately apparent, the terms“horizontal-longitudinal,” “horizontal-lateral,” and “vertical” that areused here are then to be understood mutatis mutandis, depending on theactual orientation of the X axis (advance direction), Y axis (vibrationdirection), and Z axis (transverse direction perpendicular to thesection plane).

Returning to FIG. 1, knife holder 5 is attached to the front side ofvibrating head 2, knife 6 being retained at the lower end and a cuttingedge of the knife projecting out of the knife holder. In known fashion,knife 6 is inclined toward the section plane (more precisely, toward theX axis) in order to obtain a desired sectioning result. The cutting edgeideally extends exactly parallel to the Y axis, i.e. perpendicular tothe X and Z axes. With the aid of an adjusting screw 7, knife holder 5can be pivoted about guidance axis 8. One complete rotation of theadjusting screw corresponds, for example, to a 5.3-mrad tilt of theknife (equal to a 5.3-μm change in the vertical runout with reference toa 1-mm horizontal-lateral displacement of the cutting edge).

Referring to FIG. 2, measuring head 3 comprises a light barrier in the Xdirection that is implemented by means of an IR LED as transmittingelement and an IR photodiode as detector. Optical axis 9 of the lightbarrier is shown in FIG. 2 as a dashed line. The lateral extension ofthe light barrier (as defined by the lateral extension of the LED andphotodiode plus any opening apertures present in the measuring head) isin the order of magnitude of 1 mm, and thus substantially greater thanthe vertical offset over one vibration amplitude. The knife ispositioned so that approximately 50% of the IR light is covered; thephotodiode measures the quantity of IR light propagating unimpededlybeneath the cutting edge, and thus the amount by which the light barrieris covered by the cutting edge (the knife). The purpose of compensatingfor vertical runout is to set the path of the cutting edge so that whenvibration (oscillation along the Y axis) is switched on, the amount ofcoverage changes as little as possible over one vibration period—andideally remains constant.

A limitation of light beam 9 in terms of its extension—in particular inthe Z direction, which might represent an alternative approach toenabling a more direct resolution of the Z position of the cuttingedge—was not considered because of the drastic loss of light intensity(and therefore sensitivity) associated therewith.

As already mentioned, FIGS. 1 and 2 show vibrating microtome 1 with ameasuring head installed for compensating for vertical runout. Afterknife holder 5 is adjusted, the measuring head is removed and isreplaced with the sample carrier having the sample that is to besectioned.

A control panel 10 that is shown in FIG. 3 is embodied, for example, asa separate control console that is connected to the vibrating microtomevia a connecting cable. By way of control panel 10, values such as thevibration amplitude, Z position, and (for the sample holder only) theadvance in the X direction can be set, and the result of the verticalrunout measurement can be presented on display 11 along with othernumerical values as necessary. The functions of those components of thecontrol panel that are essential for the invention is evident from thediscussion that follows; operating components that are not discussedhere serve purposes that are not of significance for the invention orare reserved for later expansion.

Electronic Control System

FIG. 4 is a block diagram of the control system of the vibratingmicrotome. The components of main control system C-1 are housed in thebody of vibrating microtome 1; in addition, control and drive componentsare also present in vibrating head 2 (box C-2 in FIG. 4) and in the bodyof measuring head 3 (electronic measurement system C-3), and in controlpanel 10 (display system C-IO).

Vibrating head control system C-2 measures the amplitude deflection ofvibrating head 2 and thus of knife holder 5. Drive is performedelectromechanically using a stationary air-core coil L1 and permanentmagnets (not shown; cf. in this regard the article by J. R. P. Geiger etal.) coupled to an aluminum base block. The base block, together withtwo laterally mounted leaf springs, forms a spring-mass system whoseresonant frequency is determined by the spring constant and the mass ofthe drive head. The vibration profile is sinusoidal to high accuracy, sothat the derived signals (unless otherwise indicated) are likewisesinusoidal. Drive current jl1 of air-core coil L1 is furnished by maincontrol system C-1. The deflection of the system is measured by means ofan IR light measurement section LS1 whose IR photodiode furnishes analternating current corresponding to the vibration and having anoverlaid DC component; this measured current js1 is amplified in asignal amplifier OP1 and converted into a voltage, the DC voltagecomponent being separated out. This AC voltage signal is amplified againin a differential line output driver OP2, outputted as non-inverted andinverted voltage signals vs1, vs2, and conveyed to main control systemC-1; the signal has, for example, a value of 1 V per mm of vibratinghead deflection. The measured vibrating head motion signal istransferred in the form of two mutually inverted signals in order tocompensate for interference occurring along the transfer path.

In main control system C-1, the two signals vs1, vs2 are converted bymeans of a differential amplifier OP3 (by calculating the difference ofthe two signals) into a control signal tp2 for the vibrating headvibration, e.g. in the form of an AC voltage signal. A sample-and-holdswitch SH ascertains the amplitude width of control signal tp2 in theform of a DC voltage signal (e.g. 1 V per mm of vibration amplitude;signal range from 0 to 3000 mV). This signal is delivered as actualvalue v1 to an amplitude regulation circuit OP4 that compares it withsetpoint v0 and, acting substantially as a PI controller, generatesdrive signal jl1 for drive coil L1 of the vibrating head at theoperating vibration frequency.

From control signal tp2, a square-wave signal trg1 is derived by meansof a zero-crossing detector OP5, and from the latter signal amaximum-minimum detector MC4 generates two control signals pklo, pkhi.Signals pklo, pkhi exhibit needle pulses at each point in time of thesignal minimum and maximum of control signal tp2. One of the signalspklo, pkhi is delivered as a trigger signal to sample-and-hold switch SHdiscussed above. Both signals pklo, pkhi are delivered to electronicmeasurement system C-3 of the measuring head and act for the latter ascontrol application signals (synchronization signals) for accuratelytimed definition of the vibration end points.

Electronic measurement system C-3 is located, for example, in the bottomof measuring head 3 and is controlled by a microcontroller MC3 thatcommunicates with main control system C-1 and with display system C-IO,and comprises for that purpose a serial module SM3 that implements aserial bus SBUS in the manner of the known RS485. During the measurementoperation, electronic measurement system C-3 is connected to maincontrol system C-1, for example, via a cable conductor (not shown inFIGS. 1 and 2) that, in addition to serial bus SBUS and the signalconductors of signals pklo, pkhi, also contains the voltage supply (15V) to the electronic measurement system.

The light barrier arrangement of the measuring head is representedsymbolically in FIG. 4 by the reference character LS2. The transmittingdiode is powered by a current supply OP6 that furnishes a supply currentwhose intensity is predetermined by microcontroller MC3 via a controlsignal tpa2. The detector diode furnishes a detector current that isconverted by a signal amplifier OP7 into a voltage signal tpa3. Inaddition, a further amplifier OP8 can be provided that serves as anamplitude scaler in the manner of a multiplier, and maps the magnitudeof voltage signal tpa3 onto a desired scale (e.g. 1 mV corresponding to1 μm of knife travel, or 1 mV per rotation of setting screw 7); scalefactor tpa1 is furnished to amplifier OP8 by microcontroller MC3. The(optionally scaled) voltage signal thereby obtained is delivered tomicrocontroller MC3 as a measured signal tpm.

Microcontroller MC3 determines the magnitude of measured signal tpm ateach of the points in time defined by control application signals pklo,pkhi; the instantaneous values thereby obtained, which correspond to thepositive and negative peak values of the oscillating signal tpm, arebuffered in digitized form and a difference between the values iscomputed and sent via serial bus SBUS to display system C-IO. There thevalue is received by microcontroller MC2 and displayed as the transverseoffset on display 11 of control panel 10. Alternatively, the twoinstantaneous peak values may be sent from microcontroller MC3 viaserial bus SBUS to display system C-IO, and a difference between thevalues may be computed by control panel microcontroller MC2 anddisplayed as the transverse offset on display 11 of control panel 10.

Returning to main control system C-1, a microcontroller MC1 is incommunication by means of a serial module SM1, via serial bus SBUS, withmicrocontrollers MC2, MC3 of control panel 10 and of measuring head 3.Microcontroller MC1 retains, for example in memory registers VA, MX, MZ,the values of the vibration amplitude (conveyed as setpoint signal v0 tocontroller OP4), sectioning feed speed, and section thickness,respectively. Using the values just recited, the motors, namely DC motorM1 for the X direction and stepping motor M2 for Z positioning, arecontrolled via respectively associated motor controllers DMC, SMC. Theadvance speed and the setpoint parameter for the Z position are setmanually on the control panel, for example using control knobs P1, P2.Actuation of one of the buttons (FIG. 3) on the control panel isdetected by control panel microcontroller MC2 in a manner known per se,and is reported via serial bus SBUS (serial module SM2) to the maincontrol system microcontroller MC1.

Vertical Runout Measurement

The procedure occurring in a vertical runout adjustment is, for example,as follows:

The electrical connection between measuring head 3 and the microtome iscreated, for example by plugging in the connecting cable (and, ifapplicable, by inputting a corresponding command on control panel 10).Readiness is indicated on control panel 10, for example by displaying“VCHECK” on display 11.

The user actuates the DOWN button on the control panel. The main controlsystem causes the measuring head to be Z-positioned into the lowestposition, and the vibrating head also moves the knife into the rearmostposition. After installation of the knife 6 (and after any manual coarsesetting of the knife inclination), clamping screw 12 is tightened.Measuring head 3 is installed on holder 4 and immobilized using clampinglever 4 a.

Once installation is complete, the user actuates the RUN button. Themain control system thereupon moves the vibrating head forward so thatknife 6 is positioned above light barrier 9 of the measuring head. Thelight barrier is still completely exposed, and in the meantime measuringhead microcontroller C-3 can usefully set the intensity of the lightbeam, via control signal tpa2, to a value at which output signal tpm isregulated to an output value that corresponds to 95% of the modulationcapability of the detector element of the light barrier. Setting theintensity at the beginning of a vertical runout adjustment compensatesfor possible interfering influences such as extraneous light,temperature fluctuations, and so on.

The measuring head is then moved upward in the Z direction into aposition in which the knife partly covers the light barrier. This isdetected by the fact that because of the occlusion by the knife, signaltpm drops to a predetermined fraction of the initial value, for example50%, with a tolerance of e.g. +/−1%. The measuring apparatus is thus ata working point at which the correlation between Z position and lightquantity is linear, and that offers the greatest possible sensitivity.

If no occlusion can be achieved, a fault exists and the measuring headis moved back down into the lowest Z position.

Once positioning in the light barrier is achieved, vibration is startedat the amplitude set on the control panel. The speed in the X directionis zero. Electronic measurement system C-3 now determines the verticalrunout as described above with reference to FIG. 4, and sends themeasurement result to microcontroller MC2 for display on the controlpanel. For example, a value “+3.4” might be displayed, which means thatthe vertical runout can be corrected with 3.4 clockwise rotations of thesetting screw (a negative value would mean a counterclockwise rotation).The value displayed could also be scaled differently, for example inμm/mm (Z vertical runout as a function of offset in the X direction).

The user acknowledges the display, for example by actuating a specificbutton such as STOP or PAUSE. Vibration of the vibrating head isstopped, if it has not already been shut off once measurement iscomplete. The user can now adjust the knife inclination. For this, he orshe loosens clamping screw 12, rotates setting screw 7 the amountindicated, and retightens clamping screw 12.

It should be noted that this adjustment operation is deliberatelyperformed manually. Although it would be an additional simplificationif, for example, a positioning motor or a piezoelement were provided foralignment of the knife inclination, experience has shown that—leavingaside the electrical and electronic complexity associatedtherewith—positioning elements of this kind would make the vibratinghead unnecessarily heavy, which would unfavorably change the vibratorybehavior of the vibrating head.

As a rule, the calibration is already sufficient after the first pass;it is nevertheless advisable to check the value by repeating themeasurement operation and, if applicable, readjusting the knifeinclination. For this, the user actuates the RUN button and theoperation presented above proceeds from the beginning. In principle, theoperation can be repeated as often as necessary, until the measurementyields a vertical runout of zero.

Because the associated Z position is set at each pass, any displacementof the Z position of the knife resulting from the adjustment is alsosimultaneously compensated for.

When adjustment is complete, the user actuates the DOWN button forconfirmation. The measuring head is moved back into the lowest Zposition and the knife is moved back. No buttons other than RUN (whichstarts a new measurement run as described above) are accepted. Thesystem now waits for the measuring head to be removed and electricallydisconnected from the microtome. A sample holder can now be installedand connected in its place; as soon as this has happened, a sectioningoperation can begin, proceeding in the familiar manner on the basis ofthe parameters inputted via control panel 10, in particular vibrationamplitude and Z position (by way of the UP and DOWN buttons).

Calibrating the Scale Factor

Calibration of the scale factor tpa1 is accomplished after input of acorresponding command on the control panel when measuring head 3 isconnected, e.g. by actuating the AUTO/MAN button. The vibrating head isthen brought into a position in which knife 6 is positioned above lightbarrier 9 of the measuring head. Measuring head microcontroller C-3 can,as described above, adjust the intensity of the light beam. Themeasuring head is then moved up in the Z direction into the position,described above, in which the knife partly covers the light barrier, anda measurement of the vertical runout is performed. The value of signaltpm is buffered by microcontroller MC3. The user is then requested, forexample by output of an instruction via the display, to change thesetting of adjusting screw 7 by exactly one clockwise rotation. When theuser has executed this rotation and indicates so, for example byactuating the RUN button, a new measurement of the vertical runout isperformed. From the difference between the two values of signal tpm,microcontroller MC3 determines the factor by which the present value ofcontrol signal tpa1 is to be corrected, performs the correspondingcorrection, and stores, in an EEPROM (not shown) provided by themicrocontroller, the new value of signal tpa1 thus obtained.

1. A measuring device and vibrating knife adapted to vibrate in adirection parallel to a section plane and substantially parallel to acutting edge of the knife, comprising: a light barrier into which thecutting edge is placed, for measurement of a transverse offset of thecutting edge during its lateral vibratory motion the light barrier beingoriented in a direction parallel to the section plane and the cuttingedge partially covering a light beam of the light barrier, wherein afluctuation over time of a measured signal (tpm) derived from the lightbarrier is used to determine the transverse offset; and an electronicmeasurement system (C-3) configured to accept at least one controlapplication signal (pklo, pkhi) that describes a time course of thevibration of the knife, and to perform a determination of the transverseoffset of the cutting edge based on values of the measured signal (tpm)at points in time determined from said control application signal (pklo,pkhi); wherein the electronic measurement system is additionallyconfigured to adjust the intensity of the light beam of the lightbarrier prior to a determination of the transverse offset with thecutting edge in a position completely outside the light barrier,establishing a utilization exceeding 90% of the modulation range of adetector element of the light barrier.